PHIVEB8ITY  OF  CALIFORNIA  POBLICATIOIU 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


THE  PRUNING  OF  CITRUS  TREES 

IN  CALIFORNIA 


BY 

ROBERT  W.    HODGSON 


BULLETIN  No.  363 

May,  1923 


UNIVERSITY  OF  CALIFORNIA   PRESS 

BERKELEY,  CALIFORNIA 

1023 


David  P.  Barrows,  President  of  the  University. 


EXPERIMENT  STATION  STAFF 

HEADS   OF   DIVISIONS 

Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

,  Director  of  Resident  Instruction. 

C.  M.  Haring,  Veterinary  Science,  Director  of  Agricultural  Experiment  Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 

C.  B.  Hutchison,  Plant  Breeding,  Director  of  the  Branch  of  the  College  of 

Agriculture,  Davis. 
H.  J.  Webber,  Subtropical  Horticulture,  Director  Citrus  Experiment  Station. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 
John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Walter  Mulford,  Forestry. 
James  T.  Barrett,  Plant  Pathology. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
L.  D.  Batchelor,  Orchard  Management. 
W.  L.  Howard,  Pomology. 
"Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 
R.  L.  Adams,  Farm  Management. 

W.  B.  Herms,  Entomology  and  Parasitology. 
John  E.  Dougherty,  Poultry  Husbandry. 

D.  R.  Hoagland,  Plant  Nutrition. 
G.  H.  Hart,  Veterinary  Science. 

L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 


THE  PRUNING  OF  CITRUS  TREES  IN  CALIFORNIA 

By  ROBERT  W.  HODGSON 


CONTENTS 

Part  I — General  Principles  page 

Introduction 489 

Physiological  Considerations 490 

Nutritional  Factors 491 

Optimum  Fruitfulness 492 

Vegetative  Growth 492 

Nitrogen  Starvation 492 

Carbohydrate  Starvation 492 

Some  Applications 492 

Definition  and  Objects 494 

Part  II — Pruning  Citrus  Trees 

Reasons  for  Pruning 495 

Complicating  Factors 499 

Bud  Mutation 500 

Polymorphism 501 

Fruit  Wood 502 

Vegetative  Growth 503 

The  "Sucker" 504 

Age  of  Fruiting  Wood 510 

Pruning  the  Young  Tree 510 

The  Navel  Orange  and  Grapefruit 512 

The  Valencia  Orange -. .• 512 

The  Lemon 513 

Pruning  the  Bearing  Tree 514 

Heading  Back  Versus  Thinning  Out 517 

Practices  to  Avoid 518 

The  Navel  Orange 519 

The  Valencia  Orange 520 

The  Tangerine 520 

Other  Orange  Varieties 520 

The  Grapefruit 520 

The  Lemon 520 

Special  Cases 521 

Changing  from  Heading  to  Thinning 521 

Pruning  the  Neglected  Orchard 522 

Rejuvenating  Decadent  Trees 523 

Deheading 524 

Dehorning 526 

Skeletonizing 526 

Pruning  Frost-Injured  Trees 527 

Pruning  Gophered  Trees 528 

Time  of  Pruning 528 

Protecting  the  Pruned  Tree 530 

Sunburn 530 

Wounds 531 

Pruning  Tools 532 

Acknowledgments 532 


FOKEWOBD 

This  bulletin,  prepared  by  Robert  W.  Hodgson,  formerly  In- 
structor in  Citriculture,  now  Specialist  in  Agricultural  Extension  and 
Farm  Advisor  of  Los  Angeles  County,  is  a  timely  discussion  of  a 
subject  of  great  interest  to  citrus  growers. 

No  method  or  methods  for  the  pruning  of  citrus  trees  can  as  yet 
be  recommended  that  are  based  upon  careful  comparative  experiments 
carried  on  over  a  period  of  years.  The  data  from  such  experiments 
as  have  been  conducted  clearly  indicate  that  severe  pruning  is 
injurious  as  compared  with  no  pruning.  They  do  not,  however, 
indicate  that  no  pruning  is  a  safe  policy  to  follow. 

Mr.  Hodgson's  discussion  and  the  methods  suggested  are  based 
upon  observational  data  accumulated  over  a  number  of  years,  together 
with  a  study  of  the  methods  pursued  by  some  of  the  best  and  most 
successful  citrus  growers.  It  should  be  understood,  however,  that 
the  methods  outlined  are  not  put  forward  as  the  final  word  on  citrus 
pruning;  more  experimental  data  must  be  secured  before  methods 
can  be  recommended  without  reservation. 

H.  J.  Webber, 

Director,  Citrus  Experiment  Station. 


PART   L  GENERAL  PRINCIPLES 


INTRODUCTION 

Of  the  many  separate  operations  which  collectively  constitute  good 
practice  in  the  growing  of  citrus  fruits,  none  is  of  more  interest  to  the 
grower  than  pruning.  There  is  probably  none  concerning  which  there 
is  so  general  a  lack  of  knowledge.  Certain  it  is  that  there  is  no 
commonly  recognized  orchard  operation  concerning  which  there  is 
less  agreement. 

One  school  of  pruners  adheres  to  the  practice  of  heavy  annual 
cutting  and  claims  markedly  beneficial  results.  Another  and  larger 
group  advises  the  practice  of  light  to  moderate  pruning  with  claims 
of  equally  satisfactory  results.  A  considerable  and  increasing  number 
recommends  confining  pruning  treatment  to  the  cutting  out  of  dead 
wood,  interfering  branches,  and  vigorous  sucker  shoots,  with  results 
that  are  stated  to  be  all  that  could  be  desired. 

The  divergency  of  opinion  among  growers  of  citrus  fruits  con- 
cerning pruning  is  so  remarkable  as  to  warrant  a  brief  analysis  of 
its  causes.  The  almost  total  lack  of  experimental  data  on  this  subject 
is  partially  responsible  for  the  situation.  The  main  contributing 
factor,  however,  has  been  the  influence  exerted  by  individual  pruners 
here  and  there,  who  through  observation  and  experience  have 
developed  rather  definite  systems.  Through  long  use  or  the  dominant 
personality  of  their  originator  certain  of  these  have  become  well 
established  in  standard  orchard  practice.  Citrus  tree  pruning 
practice  in  California  has  been  developed  largely  on  the  basis  of 
mechanical  procedure  rather  than  as  a  result  of  an  understanding  of 
the  principles  involved. 

To  the  average  grower,  pruning  is  a  purely  mechanical  operation, 
consisting  of  the  removal  of  certain  parts  of  the  plant  in  varying 
amounts,  usually  practiced  simply  as  a  matter  of  custom.  Of  recent 
years  a  wholesome  tendency  to  question  the  value  of  pruning  has 
developed,  with  the  result  that  a  considerable  number  of  growers 
have  discontinued  the  practice  until  proof  of  its  necessity  shall  be 
brought  forward.  The  fact  remains,  however,  that  few  growers  have 
given  the  pruning  problem  more  than  a  very  superficial  study,  the 
majority  confining  their  efforts  to  the  attempt  to  master  some  purely 
mechanical  system  which  is  being  used  with  reported  success  elsewhere. 


490  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Pruning  is  an  operation  which  must  be  regarded  as  having  a 
profound  influence  on  the  vital  activities  of  the  plant.  Satisfactory 
results  cannot  reasonably  be  expected  to  follow  mere  perfunctory  or 
mechanical  cutting  out  of  parts  of  the  tree.  Fundamental  to  an 
intelligent  attitude  on  the  part  of  the  grower  toward  the  pruning 
question  is  an  understanding  of  the  basic  principles  of  plant  physi- 
ology which  affect  growth  and  fruit  production. 


PHYSIOLOGICAL  CONSIDERATIONS 

It  is  a  general  axiom  of  plant  physiology  that  those  factors, 
climatic  and  otherwise,  which  favor  vegetative  growth  are  in  the  main 
antagonistic  to  fruit  production.  This  principle  has  long  been 
recognized,  although  until  quite  recently  it  was  but  poorly  understood. 
Practical  applications  of  this  principle  are  many  and  varied.  Thus 
it  is  the  common  observation  that  plants  excessively  luxuriant  in 
growth  set  fruit  but  sparsely;  in  warm,  humid  climates  trees  and 
vines  frequently  grow  to  great  size  and  have  much  foliage  but  bear 
little  or  no  fruit ;  severe  winter  pruning  of  the  type  which  favors  a 
vegetative  response  is  usually  followed  by  decreased  fruit  production. 

On  the  other  hand,  it  is  equally  true  that  those  conditions  which 
in  general  act  as  a  check  to  vegetative  growth  increase  the  tendency 
to  fruit  production.  A  decrease  in  water  supply  may  accentuate  the 
fruit-bearing  tendency;  certain  types  of  summer  pruning  tending  to 
check  vegetative  growth  appear  to  favor  fruit  production.  The  chief 
function  of  life  is  reproduction  and  fruit  trees  seem  to  be  no  excep- 
tion to  the  rule.  It  appears  that  as  long  as  conditions  favor  vegetative 
growth  the  reproductive  tendency  is  but  little  expressed  but  when 
conditions  change,  becoming  less  favorable  to  vegetative  growth,  the 
reproductive  tendency  becomes  more  and  more  apparent. 

Other  things  being  equal,  the  amount  of  fruit  produced  depends 
on  a  relation  which  exists  between  those  factors  favorable  to  vegetative 
growth  and  those  conditions  favoring  fruit  production.  This  equi- 
librium varies  considerably  for  the  same  variety  in  different  localities 
and  also  from  season  to  season  in  the  same  district.  Where  no  arti- 
ficial interruptions  occasioned  by  man  or  other  agencies  intervene,  the 
season  fluctuation  in  equilibrium  between  these  two  opposing  sets  of 
factors  is  reflected  in  the  amount  of  crop  produced. 


.Bulletin  363]         THE  pruning  OF  CITRUS  TREES  IN  CALIFORNIA  491 


NUTRITIONAL  FACTORS 

A  further  analysis  of  certain  of  the  factors  which  influence  vege- 
tative growth  and  fruit  production  is  desirable  as  a  basis  for  a 
correct  understanding  of  the  pruning  problem.  What  are  the  con- 
ditions which  favor  fruit  production  and  what  are  those  which  restrict 
it?  For  convenience  they  may  be  divided  roughly  into  two  classes, 
climatic  and  cultural.  Of  the  climatic  factors  probably  the  most 
important  are  light  intensity  and  duration,  temperature,  and 
humidity.  Up  to  a  certain  degree  of  intensity  these  factors  stimulate 
and  encourage  vegetative  growth;  beyond  that  point  they  become 
increasingly  less  favorable  to  vegetative  growth  and  more  conducive 
to  fruitfulness.  With  our  present  knowledge  we  are  unable  to  dis- 
tinguish sharply  the  point  at  which  this  transition  occurs.  It  appears 
to  vary  markedly  for  different  plants. 

Of  other  factors  affecting  the  fruit-bearing  tendency  by  far  the 
most  important  are  those  having  to  do  with  nutritional  considerations. 
The  relation  of  food  supply  to  fruit  production  and  its  importance 
to  the  pruning  problem  warrant  a  brief  review  of  the  physiological 
processes  which  constitute  normal  plant  nutrition.  The  bulk  of  the 
plant  substance,  particularly  of  woody  plants  such  as  trees,  is  com- 
posed of  carbon,  the  source  of  which  is  the  carbon  dioxide  of  the 
atmosphere.  This  is  taken  in  by  the  leaves  where,  in  the  presence  of 
the  green  coloring  matter  and  of  light,  it  is  combined  with  water  to 
form  carbohydrates  which  serve  as  the  main  source  of  energy  for  the 
processes  of  metabolism  and  growth.  Therefore,  if  the  plant  is  to 
grow  and  develop  normally,  there  must  be  a  large  and  actively  func- 
tioning leaf  surface,  a  fact  long  recognized  by  plant  physiologists  but 
still  not  sufficiently  appreciated  by  fruit  growers. 

Of  far  less  bulk,  in  fact  only  a  fraction  in  comparison,  is  that  part 
composed  of  the  constituents  taken  from  the  soil.  Of  these,  there  is 
general  agreement  among  physiologists  and  growers  to  the  effect  that 
nitrogen  is  the  most  important  because  it  is  the  one  most  likely  to 
become  deficient. 

Recent  researches  of  Kraus  and  Kraybill*  are  helpful  in  making 
clearer  important  and  practical  considerations  with  respect  to  the 
nutritive  balance  existing  in  plants.  These  researches  have  set  forth 
in  simpler  terms  the  essential  relation  of  plant  nutrition  to  the  equi- 
librium between  three  groups  of  substances;  water,  absorbed  by  the 

*  E.  J.  Kraus  and  H.  R.  Kraybill,  Vegetation  and  Reproduction  with  Special 
Reference  to  the  Tomato,  Bull.  149,  Oregon  Agricultural  Experiment  Station, 
1918. 


49c2  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

roots  and  acting  as  the  carrier  for  all  food  materials;  carbohydrates, 
manufactured  in  the  leaves ;  and  nitrogenous  compounds,  absorbed  in 
dissolved  form  in  the  soil  solution. 

Assuming  that  water  is  present  at  all  times  in  sufficient  amounts — 
a  requirement  that  is  generally  recognized  by  growers — these  investi- 
gators have  discovered  strong  evidence  that  the  fruit-bearing  pro- 
pensity is  most  intimately  associated  with  the  ratio  existing  in  the 
plant  between  the  amount  of  carbohydrates  manufactured  and  the 
nitrogenous  compounds  taken  up  by  the  roots.  On  this  basis  they 
have  been  able  to  establish  four  general  classes  into  which  may  be 
grouped  the  tendencies  of  plants  toward  vegetation  and  reproduction 
with  respect  to  the  carbon-nitrogen  ratio. 

Optimum  Fruit  fulness. — The  optimum  condition  of  fruitfulness 
of  value  to  the  orchardist  is  that  in  which  the  nitrogen  needs  of  the 
plant  are  satisfied  and  in  which  a  sufficient  surplus  of  carbohydrates 
is  manufactured  to  occasion  the  storage  of  reserve  supplies. 

Vegetative  Growth. — Tt  has  been  shown  that  where  the  nitrogen 
supply  is  ample  but  the  synthesis  of  carbohydrate  materials  sufficient 
only  to  take  care  of  growth  needs  without  providing  for  the  accumu- 
lation of  surplus  for  storage,  vegetative  growth  occurs,  accompanied 
by  the  production  of  little  or  no  fruit. 

Nitrogen  Starvation. — Where  the  nitrogen  supply  is  deficient,  even 
though  carbohydrate  surpluses  exist,  vegetation  and  reproduction  are 
both  lessened  and  a  condition  of  disease  exists,  the  characteristic 
symptoms  of  which  are  small,  yellowish  or  gray  foliage,  slender,  short, 
and  brittle  shoots  or  twigs,  and  frequently  an  abundance  of  under- 
sized buds  or  flowers  of  which  a  verv  few  mature. 

Carbohydrate  Starvation. — Even  though  the  nitrogen  supply  is 
ample,  if  the  supply  of  carbohydrates  is  deficient,  vegetative  ten- 
dencies are  repressed  and  unfruitfulness  results.  Plants  in  this  con- 
dition are  generally  of  weak,  aenemic  growth,  with  pale  yellowish 
foliage,  and  frequently  are  characterized  by  total  failure  to  produce 
flowers  or  fruit. 

SOME  APPLICATIONS 

Applications  of  these  nutritional  relations  will  at  once  occur  to  the 
observant  orchardist.    A  few  of  them  will  be  reviewed  brieflv. 

A  consideration  of  these  principles  clearly  indicates  the  dangers 
which  accompany  severe  pruning  of  health}-,  vigorously  growing  trees, 
since  such  treatment  occasions  carbohydrate  starvation.  These  prin- 
ciples also  serve  to  make  clear  the  cycles  through  which  the  tree 
normally  passes  from  the  time  of  planting  until  decadence  occurs.    In 


Bulletin  363]        THE  PRUNiNG  OF  CITRUS  TREES  IN  CALIFORNIA  493 

a  fertile  soil  the  young  tree  finds  the  nitrogen  supply  ample.  Carbo- 
hydrate synthesis  being  small,  however,  on  account  of  relatively  small 
leaf  area,  the  result  is  that  the  first  few  years  of  its  life  are  spent  in 
vegetative  growth.  As  carbohydrate  manufacture  increases  with  an 
ever-expanding  leaf  surface,  the  point  is  reached  ultimately  where 
a  surplus  accumulates  and  optimum  fruitfulness  occurs.  As  the  tree 
continues  to  develop,  if  the  nitrogen  supply  in  the  soil  is  not  main- 
tained through  fertilization,  this  element  eventually  becomes  deficient 
and  decadence  from  nitrogen  starvation  occurs.  If  the  nitrogen 
supply  is  maintained,  however,  while  conditions  exist  which  occasion 
either  a  diminution  in  leaf  area  or  a  decrease  in  photosynthetic 
efficiency,  carbohydrate  starvation  occurs  and  unfruitfulness  and 
decline  result. 

It  can  thus  readily  be  seen  why  it  is  that  pruning  during  the  early 
years  prolongs  the  period  of  vegetative  growth  and  delays  the  time 
when  the  tree  comes  into  bearing.  It  is  also  apparent  why  heavy 
pruning  of  trees  in  good  fruit-bearing  condition  tends  to  repress 
fruitfulness  and  induces  vegetation.  The  reason  also  becomes  evident 
why  old  and  deteriorating  trees  may  sometimes  be  brought  back  into 
bearing  by  means  of  pruning  treatment  which  is  of  a  character 
designed  either  to  reduce  the  carbon-nitrogen  ratio  to  the  point  where 
fruitfulness  results  or  to  eliminate  tissues  of  decreased  photosynthetic 
efficiency  and  to  substitute  therefor  new  and  more  efficient  carbo- 
hydrate-manufacturing leaf  surface.  These  considerations  also  serve 
to  explain  the  well-known  response  of  certain  types  of  decadent  trees 
to  nitrogen  fertilization. 

In  the  light  of  these  principles  it  also  becomes  readily  apparent 
why  the  navel  orange  variety  has  a  tendency  to  heavy  bloom  in  the 
interior  valley  sections  where  the  light  intensity  and  exposure  are  such 
as  to  markedly  stimulate  photosynthetic  activity.  Other  and  hitherto 
similarly  unexplainable  phenomena  at  once  become  understandable, 
such  as  the  fact  that  gophered  trees  or  trees  affected  with  trunk  or 
root  diseases  so  frequently  bear  extraordinary  crops  just  prior  to  their 
death. 

But  perhaps  the  most  important  lesson  to  be  drawn  from  these 
principles  is  their  application  to  orchard  practice  in  such  a  manner  as  to 
maintain  that  ratio  between  nitrogen  intake  and  carbohydrate  assimi- 
lation under  which  optimum  fruitfulness  occurs.  The  maintenance  of 
this  nutritional  balance  at  all  times  should  be  the  aim  of  the  grower, 
and  cultural  practices  used  should  be  analyzed  in  the  light  of  their 
relation  to  this  balance.     By  way  of  illustration  let  us  consider  the 


494  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

case  of  trees  in  a  condition  of  nitrogen  starvation  where  the  nitrogen- 
carbon  ratio  is  such  that  little  or  no  fruit  is  produced.  This  con- 
dition may  be  remedied  in  one  of  two  ways,  either  by  reducing  the 
supply  of  carbohydrates  by  means  of  pruning,  or  by  increasing  the 
nitrogen  supply  by  means  of  fertilizer  applications.  Although  either 
method  accomplishes  the  desired  end,  obviously  the  latter  practice  is 
the  correct  one  to  follow,  since  nitrogen  fertilization  makes  possible 
greater  use  of  the  carbohydrate  supplies  already  available  and  thus 
provides  for  the  production  of  a  larger  quantity  of  fruit  than  is 
possible  where  fruitfulness  is  induced  through  reduction  of  carbo- 
hydrate manufacture  by  means  of  pruning. 

DEFINITION  AND  OBJECTS 

The  removal  of  any  vegetative  plant  part  may  be  regarded  as 
pruning,  irrespective  of  the  objects  in  mind  or  the  effects  on  the  plant. 
In  general  the  function  of  pruning  is  that  of  regulating  the  growth 
of  shoots  by  determining  their  location,  kind,  and  number. 

The  two  general  objects  of  pruning  are  (1)  to  give  the  plant  a 
desired  shape,  and  (2)  to  establish  and  maintain  that  relation  between 
vegetative  and  reproductive  tendencies  which  produces  optimum 
results  for  the  grower.  The  first  object,  the  development  of  a  desired 
form,  is  of  special  importance  during  the  early  years,  and  is  usually 
referred  to  as  'training.'  The  second  object  is  materially  influenced 
by  the  interests  of  the  grower.  Objects  frequently  mentioned,  for 
which  pruning  is  believed  to  be  necessary,  include  increase  in  pro- 
duction, regulation  of  amount  of  crop,  improvement  of  quality,  con- 
venience in  harvesting,  and  the  maintenance  of  longevity.  In  the 
case  of  citrus  trees  there  is  general  agreement  that  the  pruning  prob- 
lem consists  of:  (1)  the  establishment  of  a  strong  framework  system, 
and  (2)  the  maintainance  of  a  balance  between  vegetative  vigor  and 
fruitfulness  which  is  conducive  to  the  economical  production  of 
maximum  crops  of  best  quality  while  consistent  with  the  maintenance 
of  tree  health. 

In  the  light  of  the  discussion  of  general  principles  presented,  it  may 
be  seen  that  pruning  is  but  one  of  a  number  of  operations  which  may 
be  used  by  man  in  the  regulation  of  the  nutritional  balance  in  plants. 
Pruning  should  be  looked  upon  as  fundamentally  an  operation  con- 
cerned with  i  emulation  of  food  supply  and  storage,  and  its  effects  on 
the  plant  should  be  therefore  regarded  as  the  results  of  changes  in 
environment.  Properly  used,  pruning  may  be  a  valuable  aid  to  man 
in  Lis  efforts  to  secure  profitable  fruit  crops. 


BULLETIN  363]         THE  PRUNING  OF  CITRUS  TREES  IN   CALIFORNIA  495 


PART  II.  PRUNING  CITRUS  TREES 


REASONS  FOR  PRUNING 

In  the  past  the  general  practice  in  the  priming  of  citrus  trees  in 
California  has  undoubtedly  been  too  great  severity,  and  the  economic 
waste  which  has  resulted  from  severe  pruning  has  been  very  great. 
Recognizing  this  fact,  the  growers,  particularly  the  larger  growers, 
have  in  recent  years  been  showing  a  tendency  to  prune  much  less. 
In  fact,  in  the  minds  of  some  growers,  there  is  doubt  as  to  whether 
citrus  trees  in  California  require  any  pruning  treatment  whatever. 
While  the  tendency  toward  less  pruning  is  admittedly  wholesome  and 
amply  justified  both  by  experimental  evidence  and  observational  data, 
yet  to  discontinue  pruning  altogether  is  probably  going  too  far. 
Sufficient  evidence  is  at  hand  to  warrant  the  continuance  of  light  or 
moderate  pruning  as  a  means  of  efficient  citrus  fruit  production. 

Among  the  factors  bearing  on  the  need  for  pruning  citrus  trees  in 
this  state,  the  two  of  greatest  importance  have  to  do  with  lessened 
costs  of  production  and  the  maintenance  of  tree  health  and  longevity. 
Under  favorable  conditions  of  soil  and  climate  unpruned  trees  are 
almost  certain  eventually  to  become  too  large  for  the  most  economical 
handling,  for  it  is  generally  recognized  that  there  is  a  certain  size 
of  tree  above  which  the  costs  of  production  increase  faster  than  the 
returns  from  the  additional  fruit  produced  (figs.  1,  2,  and  3).  These 
increased  costs  are  principally  those  of  harvesting  the  fruit,  which 
increases  notably  with  the  height  of  the  tree,  and  those  of  fumigation 
and  spraying,  which  increase  in  proportion  to  the  volume  of  the  tree 
and  with  large  trees  reach  an  almost  prohibitive  figure.  To  the 
owner  of  the  young  orchard,  or  even  of  the  average  bearing  orchard, 
the  problem  of  the  oversized  tree  is  not  yet  apparent.  There  are  a 
goodly  number  of  old  orchards,  however,  where  this  problem  is  a  very 
real  one,  a  problem  capable  of  solution  only  by  means  of  judicious 
pruning. 

In  practically  all  districts  unpruned  citrus  trees  become  so  dense 
that  sunlight  is  excluded  from  their  interior,  with  a  consequent  loss  of 
fruit  bearing  in  that  part  of  the  tree.  The  fruit  on  such  trees  is 
carried  mainly  on  the  outer  parts  as  a  'shell'  crop,  where  it  is  subject 
to  wind  scarring,  sunburn,  fumigation  injury,  and  other  vicissitudes 


496 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


which  result  in  a  high  percentage  of  cull  fruits.  Large  trees  with 
the  fruit  practically  all  borne  on  the  outside  are  not  desirable,  since 
the  bearing  surface  is  not  efficiently  utilized.  Smaller  trees,  kept 
reasonably  open  to  the  entrance  of  light  by  means  of  pruning,  will 
bear  as  much  or  even  more  fruit  of  distinctly  better  average  quality. 


Fig.  1. — Thirty-year  old  Valencia  trees  forty  feet  tall,  never  pruned  and  too 
large  for  economical  handling. 

While  the  reasons  for  it  are  as  yet  unexplained,  there  is  no  dis- 
puting the  fact  that  in  nearly  all  citrus  varieties  certain  vigorous 
types  of  growth  occur  which  experience  has  shown  to  be  undesirable. 
The  necessity  for  pruning  as  a  means  of  eliminating  these  is  generally 
recognized  by  growers,  who  point  to  the  'wild  tops'  and  'two  story 


Bulletin  363]         THE  PRUNING  OF  CITRUS  TREES  IN   CALIFORNIA 


497 


trunks '  of  unpruned  navel  trees,  and  the  long,  spindling,  mechanically 
weak  branches  and  decadent  tops  of  unpruned  Valencia  orange  and 
lemon  trees  as  evidence  of  what  is  likely  to  occur  where  pruning  is 
not  done.    As  a  means  of  maintaining  citrus  trees  in  good  health  over 


Fig.  2. — Thirty-year-old  lemon  tree,  never  pruned  and  nearly  thirty  feet  tall. 
Too  large  for  greatest  economy.  (Photo  by  courtesy  of  the  Citrus  Experiment 
Station.) 

a  long  period  of  time,  and  as  an  aid  in  the  production  of  satisfactory 
crops  at  a  reasonable  cost,  there  is  no  question  that  pruning  is  both  a 
necessary  and  desirable  practice. 


498 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Training  treatment  for  the  young  tree,  particularly  with  certain 
varieties,  is  necessary  if  a  strong  framework,  capable  of  holding  up 
the  heavy  weight  of  fruit  which  the  trees  will  later  carry,  is  to  be 
developed.  A  large  part  of  the  expensive  propping  and  bracing  of 
citrus  trees  now  practiced  extensively  in  certain  sections  could  be 


Fig.  3. — Old  unpruned  lemon  trees  with  branches  interlacing.     Pruning  for 
convenience  in  fumigating  is  greatly  needed. 

dispensed  with  had  the  trees  been  given  proper  training  treatment 
in  their  early  years.  In  sections  where  wind  prevalence  is  high  this 
early  training  is  of  especial  importance.  The  occasional  occurrence 
of  severe  fall  and  winter  storms  with  heavy  losses  from  breakage  in 
practically  all  citrus  districts,  however,  clearly  indicates  the  desira- 
bility of  closer  attention  to  this  phase  of  the  problem. 


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499 


COMPLICATING  FACTORS 

There  are  a  number  of  factors  bearing  on  the  pruning  of  citrus 
trees  which  have  occasioned  confusion  in  the  minds  of  growers,  the 
recognition  of  which  is  helpful  in  securing  a  proper  attitude  toward 
the  subject.  It  is  certain  that  the  problem  of  pruning  citrus  trees  is 
less  well  understood  than  that  of  pruning  deciduous  trees,  one  of  the 
reasons  being  the  fact  that  citrus  trees  are  evergreens,  a  group  of 


4        <'  V 

-   ^^3 

r'\^  W    ;,\\ 

Fig.  4. — Typical  fruit  wood  branch  in  natural  position. 


which  the  physiology  has  been  much  less  studied  than  that  of  decid- 
uous trees,  and  with  which  practical  experience  has  been  much  less 
extensive.  In  addition,  the  California  citrus  fruit  grower  is  dealing 
with  tropical  evergreens  which  are  being  grown  under  conditions 
decidedly  unnatural  to  them.  There  is  considerable  evidence  that 
citrus  trees  as  grown  in  the  arid  southwest  are  subject  to  certain 
strains,  incident  to  their  lack  of  adaptation  to  environmental  con- 
ditions, which  cause  disturbances  of  one  kind  or  another.  The  'June 
drop,'  splitting  of  the  fruits,  puffing  of  the  fruits,  'dry'  fruits,  and 
other  similar  troubles  appear  to  be  evidences  of  deficient  adjustment 
to  the  environment. 


500 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


Bud  Mutation. — The  genus  Citrus,  moreover,  appears  to  be  pecu- 
liarly unstable  and  erratic,  with  a  decided  tendency  to  degeneration 
which  is  seemingly  accentuated  under  the  climatic  conditions  of  the 
arid  southwest.  The  Washington  Navel  variety,  in  the  half  century 
it  has  been  grown  in  California,  has  given  rise  to  a  number  of  easily 
recognizable  strains,  of  which  the  best  known  are  the  Golden  Nugget, 
the  Golden  Buckeye,  the  Navelencia,  and  the  Thompson  Improved. 


Fig.  5. — Well-pruned  four-year-old  lemon  tree,  propagated  from  high  pro- 
ducing parent  tree.  Precocious  and  requires  little  pruning.  Note  the  uniform 
distribution  of  fruit. 


These  are  generally  regarded  as  having  originated  as  bud  sports, 
which  are  known  to  be  of  rather  common  occurrence  in  citrus 
varieties.*     The  characters  involved  in  mutations,  however,  may  be 

*  In  this  connection  it  may  be  well  to  point  out  that  many  of  the  peculiar 
forms  which  are  commonly  regarded  as  bud  sports  are  in  reality  chimeras,  as 
they  consist  of  purely  mechanical  mixtures  of  tissues  from  two  distinctly 
different  varieties.  Trees  of  this  character  frequently  exhibit  the  characteristics 
of  the  component  varieties  in  widely  differing  degrees  in  fruits  on  the  same 
branch.  The  origin  of  chimeras  in  Citrus  is  in  all  probability  rather  common 
as  the  result  of  shoots  developing  from  adventitious  buds  at  the  bud  union  or 
subsequent  to  top  working. 


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501 


other  than  visible  differences  in  color,  shape  or  quality  of  the  fruit, 
and  may  consist  of  such  qualities  as  earliness  and  amount  of  yield,  as 
well  as  others.  This  propensity  to  bud  sporting  and  the  careless 
propagation  methods  used  until  only  recently,  undoubtedly  bear  a 
causal  relation  to  the  generally  known  fact  that  the  majority  of  the 


Fig.  6. — Fruit  wood  branch  giving  rise  to  vegetative  'rider,'  which  should 
have  been  removed  before  reaching  the  size  shown. 


bearing  citrus  orchards  at  the  present  time  consist  of  a  conglomerate 
mixture  of  different  strains,  some  of  superior  merit  and  some  decidedly 
inferior.  Each  strain  reacts  somewhat  differently,  with  the  result 
that  the  pruning  problem  is  thereby  greatly  complicated. 

Polymorphism. — Another   confusing  factor  is  the  property  pos- 
sessed by  the  genus  Citrus,  in  common  with  certain  other  classes  of 


502  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

plants,  of  producing  juvenile  forms  of  growth,  sometimes  referred  to 
as  polymorphism.  Nearly  all  citrus  varieties  have  this  tendency, 
though  it  appears  to  vary  markedly  with  different  strains.  Little  is 
known  concerning  the  causes  of  this  phenomenon,  but  there  are  many 
growers  and  nurserymen  who  believe  that  buds  from  these  different 
growths  have  the  inherent  tendency,  at  least  under  certain  conditions, 
to  produce  the  kind  of  growth  of  the  parent  shoot.  They  do  not 
regard  this  phenomenon  as  related  in  any  way  to  bud  sporting  as 
referred  to  above,  pointing  out  that  it  would  be  the  rarest  coincidence 
if  two  bud  mutations  occurred  which  were  exactly  alike,  whereas 
many  citrus  varieties  give  rise  to  certain  easily  distinguishable  kinds 
of  growth  which  are  very  much  alike  in  all  trees  of  a  given  variety 
or  strain.  Practical  recognition  of  this  peculiar  characteristic  is 
evidenced  in  the  fact  that  many  reputable  nurserymen  and  propa- 
gators studiously  avoid  taking  buds  from  any  type  of  growth  except- 
ing the  actual  fruit-bearing  wood,  it  being  the  feeling  that  only  by  so 
doing  can  they  be  certain  that  the  best  type  will  be  perpetuated. 

A  satisfactory  explanation  for  the  phenomenon  of  polymorphism 
is  as  yet  lacking,  and  it  must  be  admitted  that  the  evidence  thus  far 
adduced  with  reference  to  claimed  inherent  differences  in  polymorphic 
forms  is  altogether  insufficient  to  prove  the  claim.  There  is  need  for 
carefully  conducted  investigational  work  with  reference  to  this  whole 
problem  and  until  such  work  is  done  we  may  not  reasonably  expect 
to  determine  this  point. 

Citrus  growers  and  pruners  in  general  recognize  two  classes  of 
growth  normally  produced  by  citrus  trees,  fruit  wood  and  vegetative 
growth,  the  latter  being  frequently  further  divided  into  several  sub- 
classes of  an  intermediate  character. 

Fruit  Wood. — Nearly  all  growers  recognize  fruit  wood  as  a  slow- 
growing,  pendant,  small-leaved,  fine-stemmed  tyipe  of  growth  which 
fruits  early  and  abundantly  (fig.  4).  Trees  propagated  from  or  made 
up  largely  of  this  type  of  wood  are  thought  to  be  slow  in  growth  but 
precocious,  bearing  god  crops  at  three  and  four  years  of  age  (fig.  5). 
This  type,  on  account  of  its  slow  growth  and  pendant  nature,  is  strong 
mechanically  and  is  little  subject  to  breakage.  It  is  a  rather  common 
opinion  that  trees  propagated  from  fruit-wood  buds  require  less  prun- 
ing than  trees  propagated  from  buds  taken  from  vegetative  types  of 
growth,  as  it  is  claimed  that  they  have  much  less  tendency  to  give 
rise  to  these  growths.  The  fruit  produced  on  such  trees  is  also  thought 
to  be  of  superior  quality,  and  is  believed  by  many  packing  house 
managers  and  growers  to  hold  up  in  storage  and  transit  better  than 
the  fruit  produced  on  other  types  of  growth. 


BULLETIN  363]         THE  PRUNING  OF  CITRUS  TREES  IN   CALIFORNIA 


503 


Vegetative  Growth. — All  other  types  of  growth  normally  found  in 
citrus  trees  may  be  regarded  as  vegetative  in  character.  Specifically, 
however,  most  growers  recognize  these  growths  to  be  vigorous,  fast- 
growing,  characteristically  upright,  with  large  leaves,  coarse-grained, 
and  with  a  characteristic  terminal  fruiting  tendency  (figs.  6,  7,  8, 
and  9).     Trees  composed  predominantly  of  vegetative  growth  seem- 


Fig.  7. — The  end  of  a  long  sucker,  showing  the  characteristic  'tasseling-out' 
and  fruiting  when  exposed  to  light. 

ingly  have  a  tendency  to  rapid  growth  and  reach  large  size  at  an 
early  age  (fig.  10).  They  are  said  to  be  slow  in  coming  into  bearing 
on  account  of  their  strong  vegetative  tendency.  Vegetative  growth 
is  weak  mechanically  because  the  rapidity  of  growth  results  in 
large,  thin-walled  cells.  The  fruit  is  borne  mainly  on  the  ends  of 
long  fishpole-like  branches  and  is  subject  to  scarring  from  wind  injury. 
This  results  in  a  decreased  percentage  of  choice  and  fancy  grades. 


504  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  weight  of  the  fruit  causes  the  long  spindling  branches  to  bend 
down  to  a  horizontal  position,  as  a  result  of  which  a  new  series  of 
similar  shoots  is  produced  which  in  turn  bear  fruit  at  the  ends  and 
bend  down.  It  is  claimed  that  while  the  amount  of  fruit  borne  by 
such  trees  may  be  large  in  the  aggregate,  the  percentage  of  saleable 
fruit  is  usually  low.  The  tendency,  especially  during  the  early  years, 
according  to  the  claims  made,  is  to  bear  coarse  fruit,  although  the 
quality  generally  improves  as  the  trees  age. 

THE  ' SUCKER'* 

Much  has  been  said  for  and  against  the  '  sucker, '  and  much  con- 
fusion exists  in  the  minds  of  growers  concerning  it.  Fundamentally, 
the  sucker  is  a  parasitic  branch  in  the  tree.  But  it  may  not  always 
remain  so ;  in  fact  if  not  removed  it  usually  eventually  becomes  self- 
supporting  and  frequently  becomes  a  valuable  branch  in  the  tree. 
During  early  life  its  leaf  area  and  exposure  to  light  are  such  that, 
like  the  young  tree  or  the  older  tree  deprived  of  a  part  of  its  leaf 
surface,  it  expends  its  energies  in  vegetative  growth  at  the  expense 
of  fruiting.  This  explains  the  fact  that  the  sucker  grows  long  and 
slender  while  shaded  by  the  outer  foliage  of  the  tree.  But  as  soon 
as  it  reaches  the  light,  carbohydrate  manufacture  increases  to  the  point 
where  normal  fruitf ulness  occurs  and  the  sucker  produces  bloom  and 
sets  fruit  (fig.  7).  Keeping  the  trees  sufficiently  open  to  permit 
moderate  light  penetration  will  do  much  to  prevent  the  growth  of 
suckers  in  citrus  trees,  causing  them  to  become  interior  fruit-wood 
branches  instead.  Suckers  may  be  induced  to  branch  and  fruit  by 
repeated  cutting,  which  accomplishes  the  slowing  down  of  the  rate  of 
sap  flow.  A  pruning  axiom  generally  accepted  among  growers  is 
that  the  best  and  most  fruit  is  produced  on  much-twisted  and 
jointed  horizontal  growth  where  the  sap  flows  slowly.  Measurements 
made  in  rapidity  of  sap  movement  in  sucker  growth  and  fruiting  wood 
tend  to  support  this  idea.  A  much  more  rapid  rate  of  sap  flow  has 
been  shown  to  occur  in  the  sucker  than  in  fruit  wood. 


*  The  word  'sucker'  as  used  here  refers  to  the  strongly  vegetative  shoots  so 
commonly  found  in  citrus  trees,  rather  than  to  the  more  exact  definition  of 
'suckers'  as  strongly  vegetative  shoots  arising  from  the  trunk  or  main  branches 
as  distinguished  from  'water  sprouts'  arising  from  the  smaller  branches  or  the 
outer  parts  of  the  tree. 


BULLETIN  363]         THE  pruning  OF   CITRUS  TREES  IN   CALIFORNIA 


505 


Fig.  8. — A  typical  'sucker'  allowed  to  run  wild  in  a  young  Washington 
navel  tree.  This  sucker  is  heavily  loaded  with  fruit,  but  is  highly  undesirable 
nevertheless. 


506 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


The  fact  that  sucker  growth  can  be  curbed  and  induced  to  fruit  is 
utilized  by  certain  schools  of  primers  who  advocate  the  use  of  sucker 
growth  for  framework  limbs,  accompanied  by  selecton  of  the  best 
fruit  wood  produced  as  a  consequence  of  vigorous  heading.      This 


Fig.  9. — Characteristic    vegetative    upright    shoot    front    a    young    Valencia 
orange  tree,  caused  by  crowding  and  lack  of  attention  to  training  treatment. 

practice  cannot  be  recommended,  however,  as  it  invariably  results  in 
the  production  of  a  number  of  strongly  vegetative  shoots  which  must 
later  be  removed,  the  production  of  which  is  a  waste  of  the  energies 
of  the  tree  (fig.  11). 


Bulletin  363]       THE  pruning  of  citrus  trees  in  California 


507 


Fig.    10. — One    extreme.      Unpruned   young    Valencia    orange    tree    showing 
'two-story'  effect.     (See  also  fig.  12.) 


508  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

It  has  long  been  recognized  that  a  certain  type  of  old  tree  deterio- 
ration is  almost  invariably  accompanied  by  the  presence,  particularly 
in  the  tops  of  the  trees,  of  large  branches  of  a  sucker  nature.  Indeed, 
in  almost  every  old  orchard  there  are  tall,  columnar-shaped  trees, 
frequently  referred  to  as  'two-story'  trees,  the  tops  of  which  are  com- 
posed of  one  or  more  large  branches  which  were  originally  suckers. 
These  trees  usually  begin  to  decline  in  vigor  and  productivity  after 
a  few  years  of  profitable  bearing.  Gradual  elimination  of  the  sucker 
growth  has  in  many  cases  been  found  to  result  in  a  return  to  vigor 
and  productivity  on  the  part  of  trees  in  this  condition. 

There  has  been  and  doubtless  always  will  be  much  speculation 
concerning  the  reasons  for  the  occurrence  of  suckers.  Abrupt  changes 
of  almost  anv  sort,  such  as  irrigation  delayed  until  the  soil  has  dried 
out,  the  application  of  considerable  amounts  of  quickly  available 
nitrogenous  fertilizers,  a  period  of  high  atmospheric  temperature, 
and  others  are  usuallv  followed  bv  the  production  of  suckers.  Bv 
far  the  most  common  cause,  however,  is  pruning  treatment  which 
disturbs  the  nutritional  balance  of  the  trees.  The  amount  of  such 
growth  is  usually  in  direct  proportion  to  the  severity  of  the  pruning 
treatment.  Trees  composed  largely  of  decadent  branches,  and  trees 
affected  by  certain  little  understood  diseases  of  the  'mottled-leaf  type, 
are  especially  prone  to  produce  suckers,  lending  weight  to  the  belief 
that  the  sucker  is  frequently  an  attempt  at  tree  renewal.  It  is  un- 
questionably true,  however,  that  trees  regularly  and  moderately 
pruned  have  much  less  tendency  to  the  production  of  sucker  growth 
than  trees  unpruned  or  spasmodically  given  severe  pruning  treat- 
ment, emphasizng  the  belief  that  the  regulation  of  light  is  probably 
a  factor  of  great  importance  in  controlling  the  production  of  suckers. 

Suckers,  therefore,  while  in  many  cases  undoubtedly  possessing 
the  same  inherent  characters  as  fruit  wood,  are  during  their  early 
life  parasitic,  almost  invariably  mechanically  weak,  and  frequently 
subject  to  early  deterioration,  for  which  reasons  they  may  be 
regarded  as  of  doubtful  utility  in  the  economy  of  the  citrus  tree.  A 
realization  of  this  fact  is  evidenced  by  the  best  primers,  who  are 
inclined  to  rigorously  exclude  them. 


BULLETIN  363]         THE  pruning  OF   CITRUS  TREES  IN   CALIFORNIA 


509 


Fig.    11. — Vigorous   upright   vegetative   growth   invariably   follows   heading 
back.    Note  the  fact  that  all  the  fruit  is  on  the  low-hanging  unpruned  branches. 


510  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


AGE  OF  FKUITING  WOOD 

There  is  a  feeling  among  citrus  growers,  supported  by  consider- 
able evidence,  that  as  fruit  bearing  wood  ages,  it  gradually  declines 
in  fruitfulness.  An  observation  that  was  very  commonly  made  follow- 
ing the  1912-13  freeze  strongly  tends  to  confirm  this  idea.  Many 
trees  which  had  to  be  rather  severely  pruned  after  the  freeze  and 
from  which  normal  crops  were  not  expected  for  several  years,  to  the 
surprise  of  growers  bore  good  crops  the  following  year  and  the  second 
year  after  the  freeze  in  many  cases  carried  more  fruit  than  had  been 
produced  for  years.  This  behavior  apparently  shows  the  desirability 
of  a  periodical  renewal  of  the  bearing  area  of  the  trees.  Unquestion- 
ably, the  cause  of  low  yields  in  the  case  of  many  of  the  older 
orchards  is  the  absence  of  vigorous,  healthy  fruiting  wood,  a  defect 
which  can  be  remedied  only  by  means  of  proper  pruning  treatment. 
Ample  evidence  is  at  hand  to  show  the  importance  of  stimulating  the 
production  of  new  interior  fruiting  wood,  it  being  generally  recog- 
nized that  after  two  or  three  years  of  fruit  bearing  this  wood  loses 
the  fruiting  habit. 

PBUNING  THE  YOUNG  TEEE 

Since  the  primary  object  of  the  citrus  grower  is  the  production 
of  fruit,  pruning  treatment  during  the  early  years  should  be  of  such 
a  character  as  to  encourage  the  development  of  a  strong  and  vigorous 
tree  with  a  large  bearing  area  which  will  fruit  at  as  early  an  age  as 
is  consistent  with  the  health  of  the  tree.  Experience  has  clearly 
indicated  that  the  accomplishment  of  these  objects  is  invariably 
accompanied  by  a  minimum  of  pruning.  It  is  certan  that  severe 
pruning  treatment  during  the  early  years  results  in  dwarfed  trees 
and  delayed  fruiting  (fig.  12). 

With  practically  all  varieties,  however,  a  certain  amount  of  train- 
ing is  necessary  if  proper  framework  formation  is  to  be  secured,  and 
attention  to  this  need  is  a  matter  of  the  greatest  importance  for  the 
future  welfare  of  the  tree.  Unless  carefully  attended  to  during  the 
first  three  years  after  planting,  the  young  citrus  tree  is  almost  certain 
to  develop  a  decidedly  poor  framework.  The  tendency  is  for  the 
main  scaffold  branches  to  arise  at  or  about  the  same  place  on  the 
trunk.  This  is  encouraged  by  the  nursery  methods  used  at  the  present 
time  in  forming  the  heads  of  the  trees.  If  the  heads  are  not  thinned 
out,  in  later  years  pinched  branches  and  weak  crotches  are  certain  to 
result.     During  the  first  two  or  three  years  in  the  orchard  special 


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511 


attention  should  be  given  to  the  selection  of  framework  branches  which 
can  grow  and  develop  so  that  later  in  life  it  will  not  be  necessary  to 
remove  them  in  order  to  prevent  crowding. 

From  three  to  four  main  scaffold  limbs  should  be  selected,  dis- 
tributed as  evenly  about  the  trunk  as  is  possible  in  order  to  secure 
balance  and  symmetry  and  distributed  vertically  over  as  much  space 


Fig.  12. — Another  extreme.     Young  Valencia  orange  tree  of  the  same  age  as 
that  shown  in  figure  10,  but  dwarfed  by  severe  pruning.     No  opportunity  to  grow. 

as  is  available  in  order  to  secure  a  maximum  strength.  Crossing 
scaffold  limbs  should  always  be  avoided.  If  they  are  selected  early 
and  those  not  desired  are  repressed  by  pinching  or  thinning,  it  is 
rarely  necesary  to  head  back  these  main  leaders.  In  case  this  becomes 
necessary  they  should  be  shortened  in  to  laterals,  if  possible.  All 
remaining  branches  should  be  left  unpruned  with  the  exception  of 
unduly  long   and   vigorous   shoots,   which  may   be  shortened   in   or 


512  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

removed  with  advantage.  A  mistake  very  commonly  made  is  the 
clipping  off  of  the  ends  or  the  removal  of  the  long  slender  hanging 
branches  on  the  lower  parts  of  the  trees.  These  should  be  allowed 
to  remain  for  the  first  few  years,  as  they  constitute  the  principal 
source  of  revenue  during  the  early  life  of  the  trees. 

When  the  framework  branches  are  established,  about  all  the 
pruning  the  young  tree  should  receive  is  the  removal  of  suckers  from 
the  trunk  and  of  unusualry  vigorous  water  sprouts  from  the  head 
together  with  the  elimination  of  crossed  or  interfering  limbs,  and, 
where  convenient,  some  attention  to  the  preservation  of  symmetry. 
Unusual  care  in  spreading  the  framework  branches  of  the  young 
citrus  tree  is  neither  necessary  nor  desirable.  In  certain  districts 
there  is  too  much  tendency  to  try  to  accomplish  this  while  the  tree  is 
in  the  formative  stage.  Many  a  flat,  spreading,  undersized  tree  could 
have  been  made  to  produce  larger  crops  by  encouraging  a  more 
upright  development  while  it  was  young,  getting  the  required  spread 
afterwards.  Occasionally  a  light  or  moderate  opening  up  where  the 
growth  is  becoming  too  dense  is  desirable.  Just  as  in  the  case  of  the 
bearing  tree,  the  tendency  in  the  past  has  been  to  prune  the  young 
tree  entirely  too  much.  It  is  certain  that  this  tendency  has  cost  the 
citrus  growers  of  California  many  hundreds  of  thousands  of  dollars 
and  especially  is  this  true  of  the  practice  long  followed  of  removing 
the  long  slender  hanging  branches  which  bear  the  major  part  of  the 
crop  during  the  first  few  years  (fig.  5).  A  good  rule  to  follow, 
especially  in  the  pruning  of  young  citrus  trees,  is  when  in  doubt, 
leave  it. 

For  the  best  results  both  in  framework  formation  and  in  conserving 
the  energies  of  the  young  tree,  it  should  be  given  attention  at  frequent 
intervals.  For  the  first  three  years  it  should  be  gone  over  at  intervals 
of  two  to  three  months. 

The  Navel  Orange  and  Grapefruit. — The  Washington  navel  orange 
and  Marsh  grapefruit,  have  very  similar  habits  of  growth  and  usually 
give  very  little  difficulty  during  the  first  few  years.  Pruning  should 
be  confined  to  the  elimination  of  the  characteristic  strong- growing 
water  sprouts  which  if  not  removed  will  result  in  ill-formed,  lopsided 
trees  (fig.  8)  and  the  removal  of  sucker  shoots. 

The  Valencia  Orange. — This  variety  has  a  much  more  vigorous 
habit  of  growth  than  the  navel  orange  or  grapefruit  and  requires 
more  attention  during  the  early  years,  if  well-formed  trees  are  to  be 
secured.  The  Valencia  orange  is  an  upright  grower  and  if  unpruned 
or  poorly  pruned  will  early  develop  into  a  tall,  columnar  or  cylindrical 


Bulletin  363]         THE  PRUNING  OF  CITRUS  TREES  IN   CALIFORNIA  513 

tree,  the  upper  part  of  which  is  composed  of  a  mass  of  long  slender, 
broom-like  shoots,  which,  as  they  begin  to  carry  fruit,  bend  down, 
pulling  the  trees  out  of  shape,  and  are  subject  to  much  breakage 
unless  propped  or  supported  by  wires  (fig.  10).  Attention  to  the 
matter  of  proper  framework  formation  is  therefore  of  especial 
importance  with  this  variety. 

The  Valencia  orange  has  a  natural  tendency  to  produce  a  vase- 
shaped  tree.  This  tendency  should  be  encouraged  by  training. 
Usually  during  the  first  season  after  planting  from  six  to  ten  or 
more  vigorous  upright  shoots  arise  from  the  original  head  of  the 
tree.  If  not  property  cared  for,  these  shoots  give  no  end  of  trouble. 
Unless  thinned  to  from  three  to  five  in  number,  they  are  certain  to 
grow  long,  slender  and  weak  (fig.  9).  While  still. small  they  should 
be  thinned,  taking  care  to  leave  only  those  with  a  good  attachment. 
These  should  be  watched  carefully  and  where  they  make  too  much 
length  growth  should  be  shortened  in  to  a  lateral  branch  if  possible, 
and  if  laterals  are  not  present,  should  be  headed  back  moderately. 
Heading  back  is  best  done  after  the  shoot  has  matured  somewhat  and 
should  not  be  done  while  it  is  soft  and  growing  rapidly.  It  is  possible, 
however,  to  stop  length  growth  in  soft  rapidly  growing  shoots  by 
pinching  out  the  tips.  These  shoots  serve  to  form  the  future  frame- 
work of  the  tree  and  should  be  given  frequent  inspection  and  attention 
during  the  first  three  years. 

Shoots  not  needed  for  framework  formation  should  not  be  pruned 
unless  too  long  and  vigorous  or  interfering  with  the  scaffold  limbs. 
The  tall,  upright  shoots  which  are  so  characteristic  of  this  variety 
present  a  puzzling  problem  to  many  pruners  and  a  rather  common 
practice  is  to  remove  them  entirely,  leaving  only  the  fine-stemmed, 
more  or  less  pendant  fruit  wood.  The  removal  of  these  shoots  is 
clearly  a  mistake  and  results  in  dwarfed  trees  and  delayed  and  light 
fruiting  (fig.  12).  If  left  alone  such  shoots  soon  'tassel  out'  at  the 
ends,  bend  down,  and  start  fruiting.  Indeed  during  the  first  few 
years  the  branches  developed  from  these  shoots  normally  carry  most 
of  the  crop.  Eventually  all  of  them  must  be  removed,  though  some 
will  remain  for  years  as  good  fruiting  branches. 

The  Lemon. — The  lemon,  like  the  Valencia  orange,  is  more  vigorous 
in  growth  than  the  navel  orange  or  grapefruit,  and  requires  much 
the  same  treatment  during  the  early  part  of  its  life.  Indeed  usually 
it  requires  even  more  care  during  this  period,  since  its  growth 
propensities  vary  so  greatly  with  different  soil  and  climatic  con- 
ditions.    Furthermore,  nearly  all  lemon  strains  are  decidedly  more 


514  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

polymorphic  in  character  than  other  citrus  varieties.  An  unpruned 
young  lemon  tree  is  almost  certain  to  develop  largely  a  rank-growing 
and  mechanically  undesirable  type  of  growth. 

Being  a  rapid  grower,  the  young  lemon  tree  should  be  given  close 
attention  with  respect  to  framework  formation.  In  districts  where 
soil  and  cHmatic  conditions  do  not  accentuate  vegetative  growth, 
selection  of  suitable  scaffold  limbs,  all  others  being  pinched  back,  will 
usually  be  found  to  give  satisfactory  results.  On  rich,  fertile  soils, 
however,  and  under  climatic  conditions  which  stimulate  growth,  it  is 
necessary  to  follow  the  treatment  recommended  for  the  Valencia 
orange  if  a  sufficiently  compact  and  sturdy  framework  system  is  to 
be  secured.  This  requires  shortening  in  the  scaffold  limbs,  which 
should  always  be  done  to  a  lateral  branch  if  one  is  available.  It  is 
easy,  however,  to  go  to  extremes  in  this  regard,  with  the  result  of 
accentuating  the  vegetative  tendency  and  delaying  the  period  when 
the  tree  should  come  into  profitable  bearing.  Especial  care  should  be 
taken  to  encourage  the  development  of  upright  scaffold  limbs,  as  other- 
wise the  weight  of  fruit  will  later  poll  these  branches  down  and  the 
final  result  will  be  a  low,  flat  tree  with  a  deficient  bearing  surface. 

Heading-in  or  tipping  shoots  not  used  for  framework  formation 
is  just  as  injurious  as  in  the  case  of  the  Valencia  orange.  Such 
branches  should  be  left  alone  as  they  constitute  the  principal  source 
of  fruit  during  the  early  part  of  the  bearing  period. 

For  the  best  results  young  lemon  trees  should  be  pruned  lightly 
every  three  months  during  the  first  four  growing  seasons  (fig.  13), 
the  operation  being  confined  largely  to  removal  of  shoots  poorly 
placed  or  undesirable  in  character. 

PEUNING  THE  BEARING  TREE 

If  properly  pruned  the  young  citrus  tree  should  come  into  profit- 
able bearing  at  from  four  to  six  years  of  age,  with  a  well  developed 
framework  system  and  good  size  and  vigor  (fig.  5).  During  the  first 
few  years  of  bearing  pruning  should  be  confined  to  the  removal  of 
suckers  from  the  trunk  and  framework  branches  and  the  elimination 
of  the  rank-growing  water  sprout  'riders'  from  the  outer  parts  of  the 
tree,  with  the  gradual  removal  of  the  first  fruiting  shoots  from  the 
lower  part  of  the  tree  as  these  fail.  Suckers  and  water  sprouts  should 
be  removed  while  still  small  and  before  seriously  draining  the  vitality 
of  the  tree.  They  should  be  taken  out  two  or  three  times  during  the 
growing  season.  For  the  next  four  or  five  years  the  bearing  tree  will 
not  require  much   in  the  way  of  pruning,  but  it  should  be  given  a 


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515 


semi-annual  inspection  and  such  pruning  treatment  as  its  condition 
demands.  From  that  time  on  the  need  for  pruning  usually  increases 
gradually  somewhat,  although  light  to  moderate  pruning  should 
always  be  the  rule.  Extremist  methods  practiced  on  healthy  bearing 
trees  are  certain  to  result  disastrously  and  moderation  in  treatment 
should  always  prevail. 


Fig.  13. — Well-pruned  two-year-old  lemon  tree. 


As  the  trees  age,  it  becomes  increasingly  important  to  regulate  the 
light  supply  if  healthy  fruiting  wood  is  to  be  maintained  in  the 
interior  parts  of  the  tree.  This  requires  thinning  out  of  the  outer 
shell  of  foliage,  which  some  pruners  accomplish  by  the  removal  of 
entire  branches  rather  than  by  a  general  thinning  of  the  fruiting 
brush.  Care  must  be  taken,  however,  not  to  leave  large  'holes,'  par- 
ticularly in  the  tops  of  the  trees,  or  injury  from  sunburn  may  result 
(fig.  14).     The  removal  of  fairly  large  branches  is  almost  certain  to 


516  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

be  followed  by  the  production  of  suckers  in  the  vicinity  of  the  cuts 
made.     These  suckers  should  be  carefully  thinned  or  shortened  in. 

Keeping  the  trees  reasonably  open  to  light  bears  an  important 
relation  not  only  to  the  maintenance  of  healthy  interior  fruiting  wood 
but  also  to  the  occurrence  of  dead  wod.  If  the  trees  are  kept  in  good 
condition  this  will  be  reduced  to  a  minimum.  Almost  invariably  a 
certain  amount  of  dead  wood  will  occur  and  it  is  desirable  to  remove 
this,  since  such  has  frequently  been  noted  to  result  in  a  general 
stimulation.  The  removal  of  dead  wood  is  not  a  matter  of  vital 
importance,  however,  since  it  apparently  undergoes  a  natural  shedding 
process  which  prevents  it  from  increasing  beyond  a  certain  point. 

As  the  trees  grow  older,  it  becomes  necessary  to  provide  for  the 
gradual  renewal  of  the  bearing  area  through  the  occasional  removal 
of  the  older  parts.  The  removal  of  these  parts  should  be  done  gradu- 
ally, however,  or  vigorous  'suckering'  is  certain  to  result.  The  first 
branches  to  lose  the  bearing  habit  are  usually  the  lower  ones  which 
are  gradually  crowded  out  by  the  new  growth  from  above.  These 
should  be  taken  out  as  soon  as  their  condition  indicates  the  passing 
of  their  usefulness.  The  removal  of  such  branches  is  generally  known 
as  'under-cutting'  (fig.  15).  It  is  not  desirable  to  take  out  bodily  old 
branches  from  the  tops  of  the  trees,  for  the  resulting  tendency  is  not 
to  produce  a  new  limb  but  to  fill  up  the  opening  by  the  growth  of 
new  wood  from  adjacent  limbs,  and  if  continued  this  process  will 
result  in  diminishing  the  bearing  surface  as  well  as  destroying  the 
symmetry  of  the  tree  (fig.  ,14).  By  far  the  better  way  is  to  remove 
several  of  the  younger  and  smaller  branches,  making  all  cuts  so  that 
the  sap  flow  is  readily  diverted  into  other  branches.  '  Stubbing '  such 
branches  should  be  studiously  avoided,  as  it  results  in  a  thicket  of 
sucker  shoots  practically  as  undesirable  as  the  original  condition. 

The  lower  branches  should  not  be  allowed  to  rest  on  the  ground. 
The  fruit  borne  on  such  limbs  is  generally  of  poor  quality  and  is  apt 
to  be  attacked  by  brown  rot.  On  the  other  hand  the  trees  should  not 
be  pruned  up  too  high,  as  considerable  bearing  area  is  thus  wasted. 
The  height  to  which  citrus  trees  are  pruned  from  the  ground  varies 
considerably  and  depends  largely  upon  the  implements  used  in  culti- 
vation. If  the  trees  are  pruned  up  a  foot  from  the  ground,  provided 
there  are  no  horizontal  branches  starting  from  the  trunk  below  that 
height,  it  is  possible  by  the  use  of  shields  on  cultivating  tools  to  work 
well  up  under  the  trees  and  at  the  same  time  to  produce  large  amounts 
of  fruit  of  good  quality  on  the  lower  branches. 


Bulletin  363]         THE  pruning  OF  CITRUS  TREES  IN  CALIFORNIA 


517 


Regular  annual  pruning  for  the  bearing  tree  should  be  the  rule, 
rather  than  the  present  all-too-prevalent  irregular  practice.  Frequent 
and  regular  treatment  tends  to  preserve  as  nearly  as  possible  the 
proper  equilibrium  between  root  system  and  parts  above  ground, 
which  if  disturbed  results  in  the  production  of  vigorous  vegetative 
growth  of  an  undesirable  character  which  must  later  be  removed. 
Further,  the  amount  of  pruning  required  to  keep  the  bearing  tree 
in  good  condition  is  reduced  to  the  minimum  by  regularity  of  treat- 
ment, with  a  consequent  saving  of  expense. 


Fig.  14. — Taking  out  the  tops  of  neglected  or  decadent  trees  is  rarely  accom- 
panied by  desirable  results.    Sunburn  is  almost  certain  to  follow  such  treatment. 

Heading  Back  versus  Thinning  Out. — Fundamentally  all  systems 
of  pruning  bearing  trees  may  be  reduced  to  the  practices  known  as 
heading  back  and  thinning  out  or  combinations  of  the  two,  and  a 
discussion  of  the  results  of  these  practices  as  they  apply  to  citrus 
fruits  is  decidedly  helpful  as  a  basis  for  rational  pruning. 

By  thinning  out  is  meant  the  entire  removal  of  a  shoot,  branch,  or 
limb,  as  contrasted  with  heading  back,  by  which  is  meant  the  removal 
of  only  a  portion  of  such  shoot,  branch,  or  limb,  leaving  another 
portion  from  which  new  growth  can  develop.  The  two  practices  have 
decidedly  different  results  in  the  case  of  citrus  trees.  Heading  back 
invariably  leads  to  greater  shoot  production  than  a  corresponding 
thinning  out  does  (fig.  11).  The  growth  response  is  localized  in  the 
vicinity  of  the  cut  made,  with  the  result  that  the  four  or  five  terminal 


518  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

buds  on  the  headed-back  shoot  are  stimulated  into  growth,  and  develop 
into  four  or  five  long,  slender,  upright  sucker  branches,  precisely 
what  the  citrus  grower  usually  does  not  desire.  Of  these  vigorous 
growing  shoots  resulting  from  heading  back  one  or  more  may  be  less 
vegetative  than  the  rest  and  may  be  utilized  for  fruit  wood.  The 
others,  however,  must  either  be  headed  back,  in  which  case  a  repetition 
of  the  same  condition  occurs,  or  they  must  be  taken  out,  thus 
representing  energy  wasted  by  the  tree. 

From  the  nutritional  point  of  view,  heading  back  changes  the 
branch  so  treated  from  the  fruiting  to  the  vegetative  condition,  or  if 
applied  to  a  branch  already  vegetative  accentuates  this  tendency. 
In  addition,  heading  back  promotes  density  and  compactness,  which 
in  turn  reduces  the  light  exposure  of  the  interior  parts. 

On  the  other  hand,  thinning  out  is  accompanied  by  less  shoot 
growth  and  is  much  less  vegetative  in  character ;  also  it  does  not 
produce  any  severe  disturbance  in  nutritional  balance,  which  might 
result  in  vegetative  growth.  Since  it  does  not  remove  so  much  stored 
reserve  and  instead  of  promoting  density  encourages  better  exposure 
to  light,  in  a  general  way  the  thinning  out  process  is  decidedly  more 
favorable  to  the  development  of  fruiting  wood  and  to  the  more  efficient 
functioning  of  that  which  is  present. 

From  this  discussion  it  is  obvious  that  both  practices  may  have  a 
legitimate  place  in  the  pruning  of  citrus  trees,  but  it  is  also  apparent 
that  heading  back  is  a  practice  which,  if  utilized  at  all,  should  be 
followed  in  the  early  life  of  the  tree  when  framework  formation  is 
the  principal  consideration,  while  thinning  out  is  the  logical  practice 
to  follow  later  when  the  problem  becomes  that  of  fruit  producton 
and  maintenance  of  tree  vigor.  As  indicated  above,  however,  with 
citrus  trees  it  is  rarely  necessary  to  resort  to  heading  back,  even  in 
the  period  of  framework  formation. 

Practices  to  Avoid. — In  addition  to  heading  back  or  'stubbing'  there 
are  certain  other  practices  which  should  be  avoided  in  the  pruning 
of  bearing  citrus  trees.  Severe  pruning  of  healthy  bearing  trees  is  to 
be  condemned  at  all  times.  There  is  ample  evidence  available  on  this 
point,  which  shows  conclusively  that  severe  pruning  of  trees  in  good 
bearing  not  only  reduces  the  yield  but  is  decidedly  repressive  of  vigor 
as  well.  These  effects  are  due  to  the  reduction  of  bearing  surface  and 
to  the  consequent  actual  carbohydrate  starvation  which  is  always 
correlated  with  marked  reduction  of  leaf  area. 

The  leaving  of  stubs  where  branches  are  removed  is  not  only  an 
evidence  of  careless  work  but  directly  encourages  the  production  of 


BULLETIN  363]  THE  PRTjninG  OF  CITRUS  TREES  IN   CALIFORNIA 


519 


clusters  of  suckers  sometimes  called  'sucker  nests.'  Cuts  should  be 
made  close  to  the  parent  limb,  as  this  will  facilitate  healing  over  and 
will  partially  overcome  the  tendency  to  sucker  growth.  Vertical  cuts 
should  be  made  where  possible,  since  they  heal  over  much  more  readily 
than  those  made  in  the  horizontal  plane. 

Shearing  off  the  outer  parts  of  the  trees  in  order  to  secure 
symmetry  is  another  practice  to  be  avoided.  In  reality  shearing  is 
merely  a  light  »form  of  heading  back  and  invariably  results  in  accent- 
uating the  production  of  upright  vegetative  shoots  which  crowd  the 
others  and  later  require  either  thinning  out  or  removal. 


Fig.  15. — The  older  worn-out  branches  are  removed  by  the  process  of  'under- 
cutting.' 

The  Navel  Orange. — The  navel  orange  has  two  rather  pronounced 
tendencies  which  must  be  guarded  against.  One  is  the  production  on 
the  main  framework  branches  of  suckers,  one  or  more  of  which  are 
apt  to  develop,  with  a  gradual  crowding  out  of  the  top  branches  to 
the  point  where  eventually  this  part  of  the  tree  is  composed  entirely 
of  sucker  growth.  Where  this  occurs,  early  decline  is  generally  the 
rule  and  the  rejuvenation  of  such  trees  is  usually  a  difficult  matter. 
The  early  removal  of  these  suckers  is  therefore  important. 

The  other  tendency  of  this  variety  in  most  districts  is  toward 
density  of  growth  with  the  resulting  death  of  the  interior  fruiting 
wood,  the  choking  out  of  the  lower  branches,  and  the  occurrence  of 
an  unusual  amount  of  dead  wood.  The  naval  orange  therefore  requires 
more  attention  with  respect  to  keeping  the  trees  open  to  light  pene- 
tration than  do  most  other  citrus  varieties. 


520  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  Valencia  Orange. — The  vigorous  upright  habit  of  growth  of 
the  Valencia  orange  makes  it  necessary  to  give  special  attention  to 
the  curbing  of  the  tall  vertical  shoots  which  characterize  this  variety. 
As  the  trees  age,  pruning  should  be  directed  toward  the  opening  up  of 
the  tops  in  order  that  the  interior  bearing  area  may  be  fully  utilized. 
Thinning  the  growth  on  the  sides  and  lower  portions  of  the  trees  is 
usually  of  much  less  importance  with  this  variety  than  with  the 
navel.  Special  attention  should  be  devoted  to  the  renewal  of  the 
fruit  bearing  wood  in  the  tops  of  the  trees,  however,  as  the  loss  of 
the  fruiting  habit  generally  occurs  first  in  this  part  of  the  tree. 

The  Tangerine. — The  tangerine  is  naturally  a  tall  upright  tree 
with  a  spreading  habit  of  growth.  It  is  invariably  a  mistake  to  attempt 
to  prune  this  variety  to  the  cylindrical  or  spherical  forms  which 
characterize  the  Valencia  and  navel  orange  varieties.  Heading  back 
and  shearing  are  especially  to  be  avoided  in  pruning  the  tangerine. 
All  that  this  variety  needs  is  the  removal  of  dead  wood  and  an 
occasional  thinning  of  the  outer  shell  to  encourage  interior  fruiting. 

Other  Varieties  of  Orange. — Usually  but  little  attention,  other 
than  the  removal  of  suckers  and  water  sprouts,  and  an  occasional 
moderate  opening  up  is  necessary  with  the  remaining  varieties  of 
oranges.  One  exception  should  be  noted  however,  viz.,  the  Mediter- 
ranean Sweet  which  has  so  dense  a  natural  habit  of  growth  as  to 
require  rather  frequent  thinning  of  the  outer  shell  of  foliage  and 
fruiting  wood.  In  its  pruning  requirements  this  variety  is  markedly 
similar  to  the  grapefruit. 

The  Grapefruit. — Ordinarily  the  bearing  grapefruit  tree  requires 
but  little  pruning.  Young,  vigorous  trees  must  be  encouraged  in  the 
formation  of  a  sturdy  framework  system,  which  may  require  some 
heading  back  of  scaffold  limbs.  The  older  trees,  however,  usually 
need  little  more  than  a  judicious  thinning  of  the  outer  parts  of  the 
tree  to  stimulate  interior  fruiting.  The  grapefruit  has  a  strong 
tendency  to  crowd  out  and  starve  the  lower  hanging  branches.  This 
may  be  overcome  partially  at  least,  by  more  attention  to  opening  up 
the  sides  of  the  trees. 

The  Lemon. — Of  a  pronouncedly  vigorous  habit  of  growth,  the 
bearing  lemon  tree  in  California  requires  considerably  more  pruning 
than  any  other  citrus  tree.  Especial  care  should  be  taken  to  keep 
the  trees  at  all  times  free  from  suckers  and  the  'rider'*  water  sprouts 
so  characteristic  of  this  variety.    The  tendency  of  the  lemon  in  certain 


*  The  term  'rider'  is  commonly  used  to  designate  water  sprouts  occurring 
on  the  upper  sides  of  branches  on  the  outer  parts  of  the  trees.  The  direction 
of  growth  of  'riders'  is  usually  at  a  right  angle  to  the  parent  branch. 


Bulletin  363]         THE  pruning  OF  CITRUS  TREES  IN  CALIFORNIA  521 

districts  and  particularly  on  fertile  soils  is  to  produce  long  spindling 
shoots  which  are  mechanically  undesirable  because  they  are  much  sub- 
ject to  breakage.  Shortening-in  of  such  branches  to  laterals  is  necessary 
to  obtain  a  satisfactory  tree.  Especial  care  must  be  taken  to  encourage 
the  formation  of  strong,  upright  framework  branches,  as  the  weight 
of  fruit  has  a  constant  tendency  to  pull  the  branches  down  and  thus 
form  a  low,  flat  tree.  The  lower  branches  as  they  are  crowded  down 
and  get  in  the  way  of  cultivating  tools  must  be  removed.  The  tops 
of  the  trees  sometimes,  though  rarely,  become  so  dense  as  to  require 
thinning.  It  is  of  special  importance,  however,  that  the  sides  and 
lower  parts  of  the  trees  be  kept  well  opened  up,  since  these  parts  of 
the  trees  if  unpruned  soon  become  so  dense  that  the  fruit  becomes 
scarred  and  picking  is  very  difficult.  A  natural  characteristic  of  the 
lemon  is  the  production  of  an  unusual  amount  of  fine  interior  fruiting 
wood,  which  must  be  periodically  thinned  to  be  efficiently  utilized. 
Maintaining  the  trees  reasonably  well  open  at  all  times  will  serve  to 
keep  healthy  new  fruit  wood  coming  on  to  replace  the  older  parts. 
These  should  be  removed  as  they  decline  and  become  unfruitful.  To 
keep  bearing  lemon  trees  in  good  condition  semi-annual  pruning  is 
necessary.  With  all  other  kinds  of  citrus  trees  annual  pruning 
together  with  the  removal  of  suckers  two  or  three  times  a  year  will 
usually  suffice  to  keep  the  trees  in  good  condition. 

SPECIAL  CASES 

The  discussion  of  pruning  in  this  publication  thus  far  has  had 
reference  to  the  young  tree  from  the  time  of  planting  until  it  reaches 
full  bearing,  assuming  that  at  all  times  it  has  received  proper  care 
and  attention.  Comparatively  few  orchards,  however,  have  been 
consistently  well-pruned  from  the  time  of  planting  until  maturity 
and  there  thus  arises  the  necessity  for  a  consideration  of  special 
pruning  problems  with  which  growers  are  frequently  confronted. 

Changing  from  Heading  to  Thinning. — In  changing  from  heading 
back  to  thinning,  it  is  necessary  to  bear  in  mind  the  fact  that  satis- 
factory results  cannot  be  secured  from  a  single  pruning  or  within  one 
season.  It  usually  requires  two  seasons  and  three  to  five  prunings  to 
successfully  accomplish  the  change.  The  most  vigorous  vegetative 
shoots  should  be  removed  first,  taking  especial  care  to  cut  them  close 
to  the  parent  limbs  in  order  to  minimize  the  subsequent  production  of 
similar  shoots.  The  less  vigorous  vegetative  shoots  should  be  left  at 
each  pruning.  If  this  practice  is  followed  the  resulting  growth  will 
become  less  and   less  vegetative  until  the  trees  are  finally  in  good 


522  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

balance  again.  Occasionally  it  will  be  necessary  to  leave  rather 
vigorous  shoots,  but  these  should  be  shortened-in  to  laterals  where 
possible.  If  laterals  -are  not  present  it  is  not  advisable  to  resort  to 
heading  back.  Allowing  the  shoots  to  go  unpruned  for  a  few  months 
will  almost  invariably  result  in  the  production  of  laterals  which  can 
be  used  later  in  the  shortening-in  process.  For  the  best  results  in 
changing  from  heading  back  to  thinning  the  trees  should  be  gone  over 
at  least  three  times  during  the  first  year.  If  vigorously  vegetative, 
the  same  number  of  primings  may  be  necessary  the  second  year, 
although  usually  such  is  not  the  case. 

Pruning  the  Neglected  Orchard. — It  should  be  stated  at  the  begin- 
ning that  the  orchard  in  which  pruning  has  been  neglected  cannot  be 
brought  into  good  condition  by  a  single  pruning.  To  do  this  requires 
several  years  and  a  number  of  primings.  An  important  consideration 
to  bear  in  mind  is  that  as  much  damage  may  be  done  by  too  severe 
priming  as  by  no  pruning  at  all.  It  is  generally  recognized  that  the 
vigor  of  response  to  pruning  is  somewhat  proportional  to  the  amount 
of  wood  removed.  Thus  in  trees  allowed  to  grow  up  to  large  suckers, 
if  all  these  are  taken  out  at  the  same  pruning,  a  rank,  vigorous  growth 
of  sucker  shoots  will  appear  in  the  place  of  those  that  were  removed. 
This  is  exactly  the  opposite  of  the  end  desired,  which  is  to  encourage 
the  production  of  the  slower-growing  fruit  wood. 

It  is  therefore  a  cardinal  principle  in  pruning  neglected  trees  to 
regulate  the  amount  of  wood  removed  so  as  to  distribute  the  response 
as  uniformly  as  is  possible  without  localizing  it.  To  accomplish  this 
requires  a  lighter  and  more  judiciously  distributed  priming  than  is 
usually  given  to  such  trees.  In  the  case  of  trees  containing  a  number 
of  old  sucker  limbs,  not  all  should  be  cut  out  at  the  same  pruning,  but 
only  one  or  two,  according  to  their  size  and  location.  Care  should  be 
taken  to  go  over  the  new  growth  produced  in  response  to  the  pruning, 
thinning  it  out  where  necessary  and  taking  out  all  vigorously  vege- 
tative shoots.  If  this-  practice  is  followed,  it  is  possible  to  bring  trees 
where  pruning  has  been  neglected  back  into  good  condition  within 
two  or  three  years. 

In  the  case  of  badly  neglected  trees  a  pruning  program  that  can 
be  recommended  is  about  as  follows :  The  first  priming  should  consist 
in  removing  a  few  of  the  sucker  limbs  and  cutting  back  some  of  the 
remainder  to  smaller  laterals.  The  second  pruning,  a  few  months 
later,  should  consist  in  the  removal  of  interfering  limbs  and  of  the 
worst  of  the  sucker  limbs  left  at  the  previous  pruning,  together  with 
a  thinning  out  and  selection  of  the  new  growth  produced  in  response 
to  the  previous  treatment.     Subsequent  primings  should  be  similar 


BULLETIN  363]  THE  pRUNING  OF  CITRUS  TREES  IN   CALIFORNIA 


523 


to  the  second.  Where  such  a  program  is  used,  within  two  or  three 
years  the  tendency  to  the  production  of  sucker  shoots  will  have  been 
largely  overcome  and  the  old  and  decadent  parts  eliminated,  both 
these  results  being  accomplished  without  seriously  disturbing  the 
equilibrium  of  the  tree  and  causing  a  waste  of  its  energies  in  the 
production  of  undesirable  growth. 


Fig.  16. — Sparse  weakly  growth  in  the  tops  of  the  trees  usually  indicates 
decline. 


Rejuvenating  Decadent  Trees. — The  bringing  back  into  vigor  of 
the  many  old  citrus  groves  which  are  rapidly  failing  in  productivity 
and  health  constitutes  a  serious  problem  confronting  California  citrus 
growers.  As  indicated  in  Part  I,  there  are  two  principal  causes  for 
the    decline    of    fruit    trees,    nitrogen    starvation    and    carbohydrate 


524  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

starvation.  Both  are  fairly  common  in  the  citrus  orchards  of  Cali- 
fornia. Of  the  two,  however,  nitrogen  starvation  is  undoubtedly 
much  the  more  prevalent. 

The  characteristics  of  decadent  citrus  trees  are  well  known  to  many 
growers  but  it  may  be  worth  while  to  briefly  enumerate  them.  In 
the  order  in  which  they  usually  appear  they  are  (1)  increase  in  the 
percentage  of  small-sized  fruit  (in  the  case  of  lemons  accompanied 
by  an  increase  of  tree  ripe  fruit)  ;  (2)  decrease  in  total  yield;  (3) 
dwarfed  foliage,  starting  in  the  tops  of  the  trees  (fig.  16)  ;  (4)  super- 
abundance of  weak  or  leafless  fruit  spurs;  (5)  heavy  production  of 
weak  blossoms,  mostly  abortive;  and  (6)  gradual  development  of 
chlorosis. 

Whether  the  specific  cause  of  decline  be  nitrogen  starvation  or 
carbohydrate  starvation,  rational  pruning  treatment  may  frequently 
be  utilized  with  advantage  in  bringing  the  trees  back  to  productivity. 
In  the  case  of  the  former,  the  reduction  of  carbon  assimilation  accom- 
plished by  pruning  reestablishes  the  proper  balance  between  nitrogen 
intake  and  carbon  intake  for  optimum  fruitfulness.  Where  the  cause 
is  carbohydrate  starvation,  proper  pruning  will  tend  to  reestablish 
the  correct  balance  for  fruitfulness  by  relieving  the  tree  of  inefficient 
and  decadent  tissues  and  substituting  therefor  new  and  more  efficient 
tissue.*  In  the  case  of  deficiency  in  nitrogen  supply,  however, 
increased  fertilization  with  this  element  is  undoubtedly  the  more 
rational  method  to  use  in  the  rejuvenation  process,  because  it  restores 
the  nutritional  balance  by  the  addition  of  the  deficient  element,  thus 
increasing  the  possibilities  of  production,  while  restoring  the  balance 
by  subtraction  merely  utilizes  the  existing  possibilities. 

There  is  little  agreement  among  growers,  however,  as  to  the  best 
methods  of  pruning  to  use  in  the  bringing  back  into  vigor  and  pro- 
ductivity of  decadent  trees.  A  brief  discussion  of  this  subject  seems 
desirable. 

Deheading. — A  method  used  in  certain  of  the  older  citrus-growing 
sections  of  the  world,  notably  Brazil,  Spain,  and  Italy,  is  the  periodic 
renewal  of  the  tops  of  the  trees  by  deheading,  and  this  method  has 
been  advocated  by  certain  investigators  and  practiced  by  a  few 
growers  in  California.  It  is,  however,  open  to  criticism  of  sufficient 
weight  to  have  discredited  it  in  the  minds  of  the  majority  of  growers. 


*  In  all  probability  the  production  of  sucker  growth  so  characteristic  of 
declining  trees  is  a  natural  means  of  restoring  the  carbohydrate  balance  by 
the  activities  of  the  new  and  vigorous  leaves  on  such  shoots,  which  tjius  tend 
naturally  to  renew  the  tree. 


Bulletin  363]         THE  pruning  OF  CITRUS  TREES  IN  CALIFORNIA 


525 


Tree  renewal  by  decapitation  is  an  extreme  method  and  felt  to  be 
unnecessarily  severe.  Entire  removal  of  the  tops  of  the  trees  has  a 
serious  weakening  effect  on  the  root  system,  the  vigor  of  the  resulting 
shoots  being  due  merely  to  their  diminished  number.  If  not  properly 
cared  for  and  given  frequent  follow-up  treatment  these  are  certain 


Fig.  17. — Extreme  skeletonization  as  applied  to  old  Valencia  tree.     (Photo 
by  courtesy  of  Mr.  E.  S.  Lewis.) 


to  grow  vegetatively  for  several  years  and  to  result  in  the  formation 
of  a  very  undesirable  framework.  In  addition  the  large  wounds  made 
rarely,  if  ever,  heal  over  properly.  Moreover,  this  severe  treatment 
delays  profitable  bearing  for  a  period  of  five  or  six  years.  This 
method  never  made  any  great  appeal  to  California  citrus  fruit  growers 
and  for  the  reasons  given  as  well  as  others  is  rapidly  losing  favor. 


526  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Dehorning. — A  modified  and  less  severe  treatment,  known  as 
dehorning  in  which  a  part  or  all  of  the  primary  and  secondary 
scaffold  branches  are  left  hut  cut  back  to  stubs  a  foot  or  more  in 
length  has  also  been  used  to  some  extent.  This  method,  however, 
differs  little  if  at  all,  except  in  severity,  from  the  head  removal  system 
and  for  the  same  reasons  is  losing  favor. 

Skeletonizing . — A  much  more  rational  method,  and  one  which  has 
apparently  given  quite  satisfactory  results  in  the  rejuvenation  of 
old  trees  is  that  best  described  as  skeletonizing.  In  this  method  the 
entire  framework  system  of  the  tree  is  left,  except  in  cases  where  too 
many  scaffold  limbs  were  allowed  to  develop  originally,  in  which 
event  one  or  more  of  these  are  removed  entirely.  All  crossed  limbs 
and  unnecessary  leaders  are  removed,  and  what  remains  is  the  simplest 
possible  skeleton  on  which  to  build  an  entirely  new  fruiting  system 
(figs.  17,  18,  and  19).  The  height  and  diameter  of  the  tree  are 
reduced  little,  if  at  all,  but  all  the  small  branches  and  shoots  are 
removed.  'Stubbing'  or  heading  back  to  branches  larger  than  an  inch 
or  so  in  diameter  is  studiously  avoided. 

Numerous  cases  where  old  orchards  have  been  pruned  by  this 
system  have  shown  that  within  three  to  four  years  the  trees  have 
entirely  recovered,  are  in  good  vigor,  and  yielding  heavier  crops  than 
ever  (figs.  17  and  18).  The  degree  of  severity  with  which  this  system 
should  be  applied  varies  somewhat  and  obviously  depends  upon  the 
extent  of  decadence  existing  at  the  time  the  pruning  treatment  is  used. 

All  things  considered,  the  skeletonizing  system  of  pruning  for  tree 
rejuvenation  appears  to  be  the  most  rational  as  well  as  successful  for 
a  number  of  reasons.  This  treatment  distributes  the  pruning  response 
throughout  the  entire  framework  instead  of  localizing  it  in  certain 
parts  of  the  tree.  It  is  generally  recognized  that  the  response  to 
pruning  treatment  is  largely  confined  to  the  region  where  the  removal 
of  parts  has  occurred.  Thus  taking  out  or  heading  back  branches  in 
the  tops  of  the  trees,  a  practice  much  used  in  the  past,  does  not 
stimulate  growth  in  the  rest  of  the  tree,  but  results  in  a  vigorous 
growth  of  vegetative  shoots  on  the  branches  near  the  cuts  and  necessi- 
tates frequent  subsequent  thinning. 

Since  the  response  is  widely  distributed  by  this  system  of  pruning, 
the  amount  of  follow-up  work  is  greatly  reduced.  Moreover,  the 
growth  resulting  is  not  markedly  vegetative,  and  shortly  becomes 
fruitful,  a  condition  greatly  desired  by  the  grower  who  is  naturally 
anxious  to  have  the  trees  come  back  into  bearing  as  soon  as  possible. 
While  this  system  necessitates  more  cuts  and  requires  more  time  and 


Bulletin  363]         THE  pruning  OF  CITRUS  TREES  IN  CALIFORNIA 


527 


is  consequently  more  expensive  than  the  other  systems  described,  for 
the  reasons  given,  it  appears  to  be  preferable  to  them. 

Pruning  Frost-Injured  Trees. — The  subject  of  pruning  frost- 
injured  trees  is  one  of  periodic  interest  to  California  citrus  growers. 
At  present,  comparatively  little  reliable  information  is  available  on 
this  subject.  Test  plots  now  under  observation  where  different  prun- 
ing treatments,  varying  in  method  and  in  degree,  are  being  used,  will 
probably  furnish  this  much-needed  information  within  a  few  years. 


Fig.  18. — The  same  tree  as  in  figure  17  two  seasons  later,  showing  remark- 
able recovery.     (Photo  by  courtesy  of  Mr.  E.  S.  Lewis.) 

There  is  ample  evidence  available,  however,  to  demonstrate  clearly 
that  under  most  conditions  of  injury  the  safest  procedure  during  the 
season  following  the  injury  is  to  discontinue  all  pruning  treatment. 
It  is  certain  that  pruning  is  inadvisable  for  some  months  after  a 
freeze,  since  it  is  ordinarily  three  to  four  months  before  it  is  possible 
to  determine  what  the  extent  of  the  injury  is.  Until  this  can  be 
definitely  determined,  pruning  treatment  is  merely  a  guess;  it  may 
be  too  severe,  or  it  may  not  extend  back  far  enough. 


528  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Where  the  injury  has  been  appreciable  and  extends  beyond  the 
mere  loss  of  foliage  to  the  death  of  fruit-bearing  wood  the  effect  of  a 
freeze  is  very  similar  to  that  of  heading  back,  and  the  response  is 
vegetative  growth.  To  prune  at  this  time  merely  accentuates  the 
situation  and  instead  of  aiding  in  restoring  the  balance  between  root 
system  and  top  only  further  aggravates  the  lack  of  equilibrium.  It 
therefore  appears  desirable  to  discontinue  pruning  until  the  trees 
have  recovered  from  the  injury,  and  experience  has  clearly  shown 
the  wisdom  of  this  practice.  Usually  full  recovery  from  the  injury 
is  not  made  until  rather  late  in  the  season,  at  which  time  pruning  is 
inadvisable  as  it  increases  the  danger  from  frost  injury  the  following 
winter.  For  these  reasons  it  appears  desirable  to  delay  pruning  frost- 
injured  trees  until  a  full  season  has  elapsed. 

Pruning  Gophered  Trees. — Pruning  treatment  for  gophered  trees 
is  determined  by  the  extent  of  injury,  the  period  elapsing  since  the 
injury  occurred,  and  remedial  treatment  provided.  Where  trees  are 
practically  girdled  remedial  treatment  either  by  inarching,  bridge 
grafting,  or  other  means  is  necessary  if  they  are  to  be  saved.  If  such 
treatment  is  provided  immediately  no  pruning  is  required. 

If  the  injury  is  not  extensive  and  natural  recovery  is  relied  upon, 
a  moderate  thinning  of  the  top  is  desirable  in  order  to  restore  as 
nearly  as  possible  the  balance  existing  between  the  root  system  and 
the  parts  above  ground. 

If  the  injury  is  not  detected  for  some  time,  and  the  condition  of 
growth  indicates  a  serious  disturbance,  it  appears  to  be  desirable  to 
remove  a  part  of  the  top,  regardless  of  the  extent  of  the  injury.  The 
character  of  the  remedial  treatment  to  be  later  applied,  however,  is 
determined  by  the  extent  of  the  injury.  Where  the  injury  is  extensive 
and  the  condition  of  the  trees  decidedly  weak,  skeletonizing  may  be 
necessary  if  they  are  to  be  saved.  Where  such  is  the  case,  however, 
it  is  questionable  whether  the  trees  are  worth  saving,  even  though  it 
be  possible  to  do  so. 

TIME  OF  PEUNING 

Few  data  are  available  concerning  the  effects  of  pruning  citrus 
trees  at  different  seasons  of  the  year.  So  far  as  is  now  known  the 
response  of  the  trees  is  practically  the  same  regardless  of  the  season 
when  the  pruning  is  done. 

It  is  generally  recognized,  however,  that  pruning  in  late  summer 
or  early  fall  is  undesirable,  since  it  encourages  vegetative  growth  and 
delays  the  maturing  of  the  wood,  thus  rendering  the  trees  more 
susceptible  to  injury  from  frost  (fig.  20). 


BULLETIN  363]         THE  PRUNING  OF  CITRUS  TREES  IN   CALIFORNIA 


529 


The  bark  on  the  shaded  limbs  of  citrus  trees  is  very  susceptible  to 
sunburn  and  equally  so  to  injury  by  low  winter  temperatures.  This 
renders  it  decidedly  hazardous  to  do  any  bulk  pruning,  where  large 
limbs  are  removed,  during  the  hot  summer  season  or  during  the  winter 
period  when  low  temperatures  are  likely  to  occur.  For  security  such 
treatment  should  be  given  during  the  spring  or  early  summer. 


Fig.  19. — Skeletonization  as  applied  to   thirty-five-year-old  decadent  lemon 


tree. 


With  bearing  trees  reasons  of  convenience  largely  determine  the 
time  of  pruning.  The  most  important  reason  is  the  presence  of 
maturing  fruit  on  the  trees.  Thus  it  is  the  common  practice  to  prune 
the  trees  after  the  crop  is  harvested  in  the  case  of  the  navel  orange 
and  grapefruit,  or  when  there  is  the  least  amount  of  saleable  fruit  on 


530 


UNIVERSITY    OP    CALIFORNIA EXPERIMENT    STATION 


the  trees  in  the  case  of  the  Valencia  orange  and  the  lemon.  The 
pruning  of  navel  oranges  is  therefore  largely  done  during  late  winter 
and  early  spring;  Valencia  orange  and  lemon  pruning  during  the 
summer  months. 

Pruning  neglected  bearing  trees  and  pruning  for  tree  rejuvenation 
should  be  done  during  the  late  winter  and  spring  months,  although 
sometimes  it  can  be  done  with  advantage  in  early  summer. 


Fig.  20. — Old  lemon  trees  severely  pruned  in  late  fall.     Properly  protected 
against  sunburn,  but  so  badly  injured  by  frost  as  to  necessitate  removal. 


PKOTECTING  THE  PRUNED  TREE 

Sunburn. — The  bark  of  citrus  trees  is  very  susceptible  to  injury 
from  sunburn  and,  where  the  injury  is  extensive,  recovery  is  exceed- 
ingly rare.  A  few  hours'  exposure  to  the  hot  sun  may  be  sufficient 
to  cause  irreparable  injury.  Protecting  the  trees  against  sunburn, 
therefore,  is  a  matter  of  great  importance  to  the  citrus  grower. 

In  all  cases  where  pruning  results  in  exposing  framework  branches 
to  direct  sunlight,  even  though  only  for  a  few  hours  during  the  day, 
such  limbs  should  be  thoroughly  coated  with  whitewash.  White- 
washing is  especially  important  where  heavy  pruning  incident  to  tree 
rejuvenation  is  done  (fig.  20 ). 


BULLETIN  363]         THE  PRUNING  OF  CITRUS  TREES  IN   CALIFORNIA  531 

There  are  many  satisfactory  whitewash  formulas.  One  of  the 
simplest  and  best,  however,  is  as  follows: 

Quicklime  (unslacked  or  rock  lime) 7  pounds 

Powdered  sulfur 2  pounds 

Salt  1  pound 

Slake  together,  using  sufficient  water  to  accomplish  uniform  and 
thorough  slaking;  then  dilute  to  the  desired  consistency.  Allow  to 
stand  overnight  before  using. 

Wounds. — Wounds  made  in  pruning  afford  opportunity  for  the 
entrance  of  decay-producing  fungi  which  under  favorable  conditions 
occasion  heart  rot  of  the  trees  and  ultimately  cause  serious  disturb- 
ances and  even  death.  In  the  dry  interior  valleys,  the  danger  of 
decay  is  not  so  great,  although  always  present.  It  is  desirable,  there- 
fore, both  as  a  means  of  protection  as  well  as  to  encourage  rapid 
healing  over  to  cover  all  cut  surfaces  greater  than  two  inches  in 
diameter  with  some  antiseptic  dressing.  The  application  of  a  dis- 
infectant dressing  prior  to  the  use  of  a  wound  covering  is  advocated 
by  some,  while  others  maintain  that  just  as  good  results  are  experi- 
enced without  disinfectants.  In  case  such  are  desired  the  best  that 
can  be  recommended  at  the  present  time  is  mercuric  cyanid  dissolved 
in  equal  parts  of  ethyl  alcohol  and  water  in  the  proportion  of  one 
part  of  cyanid  to  1000  parts  of  the  alcohol-water  solution.  Whether 
disinfectants  are  used  or  not  the  wounds  should  be  allowed  to  dry 
thoroughly  before  applying  the  wound  dressing. 

There  are  several  requirements  for  a  satisfactory  wound  dressing, 
the  most  important  of  which  are  cheapness  and  permanency.  Other 
requirements  include  lack  of  penetration,  and  elasticity.  Whitewash, 
bordeaux  paste,  lead  paints,  and  varnishes  or  shellacs  do  not  satis- 
factorily meet  these  requirements,  since  they  soon  dry  out  and  lose 
their  protective  qualities.  Grafting  wax.  while  excellent  in  many 
respects,  is  too  expensive  at  present  prices  for  beeswax. 

The  most  promising  class  of  available  materials  are  the  asphaltum 
compounds.  Asphaltum  roof  paint  has  been  used  by  some  with  good 
results.  Grade  B  asphaltum,  a  liquid  form,  is  popular  with  some 
growers.  A  liquid  preparation  made  by  dissolving  asphaltum  in 
gasoline  or  distillate  is  recommended  by  certain  authorities.  Boiled 
coal  tar  has  been  used  and  is  still  used  to  some  extent,  and  there  are 
a  number  of  commercial  preparations  on  the  market.  Probably  the 
best  dressing  for  the  purpose  is  still  to  be  worked  out. 


532  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  important  thing,  however,  is  to  make  certain  that  all  wounds 
of  an  inch  or  more  in  diameter  are  well  coated  with  some  dressing 
which  is  renewed  occasionally  when  this  is  found  by  inspection  to  be 
necessary. 

PRUNING  TOOLS 

Without  good  tools  satisfactory  pruning  is  impossible  and  much 
poor  pruning  is  directly  due  to  a  lack  of  recognition  of  this  fact.  A 
wide  varietj^  of  tools  may  be  found  in  use  by  citrus  pruners,  but 
experience  has  indicated  that  the  following  are  best  adapted  to  their 
needs : 

1.  A  short,  light,  self-supporting  stepladder. 

2.  One  pair  strong  leather  gloves. 

3.  One  strong  pair  six-inch  hand  shears.* 

4.  One  leather  case  for  these  shears,  attached  to  belt. 

5.  One  fourteen-inch  bracket  sawf   (six  extra  blades). 

6.  One  curved  blade  folding  pruning  saw  of  the  pull  type.j; 

7.  Bucket  of  wound  compound  with  brush. 

The  saw  in  most  general  use  is  a  pull  saw  with  a  blade  eight  to 
twelve  inches  long,  the  cutting  edge  of  which  has  a  curved  shape. 
There  are  many  types  of  this  saw  on  the  market ;  a  number  of  these, 
however,  have  too  much  curve  to  the  blade.  For  the  removal  of  large 
crowding  limbs  these  are  not  well  adapted  and  the  saw  most  commonly 
used  is  one  of  the  bracket  pattern  with  a  twelve  to  fourteen-inch 
swivel  blade,  sometimes  known  as  the  California  pruning  saw.  Some 
pruners  carry  both  types  of  saws  in  their  outfits,  but  the  usual  outfit 
contains  only  one  saw. 

Hand  lopping  shears  have  fallen  into  disfavor  on  account  of  their 
tendency  to  split  the  limbs  below  the  point  where  the  cut  is  made. 

ACKNOWLEDGMENTS 

The  author  desires  to  express  .his  appreciation  to  Dr.  L.  D. 
Batchelor,  Dr.  II.  S.  Reed,  and  Dr.  II.  J.  Webber,  all  of  the  Citrus 
Experiment  Station  at  Riverside,  and  to  Professor  W.  P.  Tufts  and 
Dr.  W.  L.  Howard  of  the  Agricultural  Experiment  Station  at  Berkeley 
for  many  helpful  criticisms  and  suggestions,  and  especially  to  Pro- 
fessor F.  T.  Bioletti  for  his  careful  revision  of  the  manuscript  and 
his  interest  in  its  preparation. 

*  Riescr  pattern  preferred. 

t  Bishop  pattern  recommended. 

t  Choice  between  Tyler  and  Atkins  patterns. 


